The invention concerns testing of gas turbine engines and, more specifically, recovery of heat which is ordinarily wasted in the testing process. The invention particularly concerns recovery of waste heat without interfering with the exhaust of an engine under test. Such interference is to be avoided because it can produce undesired back-pressure in the exhaust.
Gas turbine engines, such as those used in aircraft and naval vessels, are tested in ground-based centers called test cells, which are generally, but not always, located at the site where the engines are manufactured.
Engines under development are tested during various stages of the development process: a single prototype can be tested multiple times. Also, once an engine design is fully developed, each individual engine manufactured according to that design can be tested after its completion.
The testing process consumes large amounts of jet fuel, representing a large consumption of energy. A simple example will illustrate the amounts of fuel involved.
In the gas turbine art, specific fuel consumption is defined as the amount of fuel consumed per hour in order to develop one pound of thrust. Engines producing thrust of 40,000 pounds are commonly produced, although engines producing higher and lower thrusts are available. An engine in the 40,000 pound class would thus consume 40,000xc3x970.25, or 10,000 pounds of fuel per hour. Since jet fuel weighs roughly 6 pounds per gallon, 10,000 pounds of fuel represents about 1,667 gallons.
Therefore, testing the engine in question at rated thrust for one hour consumes 1,667 gallons of fuel. By comparison, many automobiles consume around three to five gallons of gasoline per hour.
In addition, not all the energy contained in the fuel is converted by the gas turbine engine into mechanical work. Much is lost as heat. The approximate size of this loss will be considered.
The gas turbine engine is based on the Brayton, or Joule, cycle, which is characterized by constant-pressure combustion. Efficiency is defined as (net work output)/(heat supplied). For an ideal Brayton cycle, the cycle efficiency can be shown to be
Efficiency=1xe2x88x92(1/r)**[(gammaxe2x88x921)/gamma]
wherein
r is the pressure ratio,
gamma is the ratio Cp/Cv, namely, constant-pressure heat capacity to constant-volume heat capacity, Cv, of the working fluid, and
the dual asterisks, **, represent exponentiation.
FIG. 1 illustrates efficiency plotted for a gamma of 1.4, which is the gamma of air. Clearly, a theoretical maximum efficiency is less than sixty percent for the pressure ratios shown.
At this assumed efficiency, only 60 percent of the heat content of the fuel is utilized to produce mechanical work. The rest is lost as waste heat. The invention proposes stratagems to recover this wasted heat.
In one form of the invention, waste heat produced in the testing process of a gas turbine engine is recovered. As a specific example, in test cells, exhaust from the engine is ducted to the atmosphere through exhaust pipes. Water is applied to the exhaust pipes, to prevent them from being damaged by the heat of the exhaust. This water turns to steam, and is vented to the atmosphere.
In one form of the invention, water tubes are wrapped around, or within, the exhaust pipes. Heat is extracted from the heated water within the tubes and stored.